Binders suitable for the formulations of cathodically depositable decorative single coat paints are described, for example, in DE-OS No. 23 25 177. In that document acrylic copolymers carrying glycidyl groups are reacted with 0.1 to 1.0 equivalents of a secondary amine per glycidyl group equivalent. The binders obtained crosslink essentially via the unreacted glycidyl groups. These groups, however, are not stable in an aqueous solution for any prolonged period of time, which causes a reduction in the curing characteristics of the binder in the course of time and renders the paint useless.
EP No. -Al-01 31 125 discloses the use of crosslinking components based on beta-hydroxyalkylester group containing compounds to crosslink amine adducts of glycidyl group containing copolymers.
AT-PS No. 372,099, corresponding to U.S. Pat. No. 4,458,054, discloses the use of malonic acid esters as crosslinking agents for cationic binders. This document also refers to the possibility of using the malonic acid esters with copolymers containing cationic groups.
AT-PS No. 379,602, corresponding to U.S. Pat. No. 4,523,007, also describes crosslinking agents based on malonic acid esters which react by transesterification and transamidation. According to the disclosure, CH-acidic compounds carrying at least one ester group are reacted with a carbonyl compound in a KNOEVENAGEL-type reaction, and the resulting alpha-disubstituted alkylidene compound is polymerized to an oligomeric compound with a molecular weight of from about 300 to 6000. Optionally, the crosslinking agent can be partially pre-condensed with a basic resin used in the binder system.
According to the known disclosures, the crosslinking agents are only mixed with the polymers. It has been found, however, that when using acrylic copolymers as the base resin, probably due to severe incompatibility, resin blends and pigment pastes separate into two phases after only short storage times. Satisfactory re-dispersion is largely uneconomical. A further disadvantage results from surface defects in the films which cannot be remedied in most cases.
It has now been found that binders for paint systems based on acrylates which crosslink through transesterification or transamidation and are suitable for cathodically depositable paints to provide excellent performance, can be obtained through the judicious selection of the binder components.